The present invention relates to a nuclear fuel assembly including a fuel bundle and a lower tie plate for supporting the fuel bundle and particularly relates to a filter plate resting on a lower tie plate grid for screening debris from the flow of coolant into the fuel bundle.
In a nuclear reactor, liquid coolant/moderator flows into the reactor core from the bottom and exits the core as a water/steam mixture from the top. The core includes a plurality of fuel bundles arranged in vertical side-by-side relation, each containing a plurality of fuel rods. The fuel bundles are each supported on a lower tie plate. The tie plate typically includes an upper grid, a lower inlet nozzle and a transition region between the inlet nozzle and the grid whereby coolant water entering the inlet nozzle flows through the transition region and through the grid generally upwardly and about the individual fuel rods of the fuel bundle supported by the lower tie plate.
Over time, debris accumulates in the reactor and can result in fuel bundle failures in the field by debris fretting through the fuel rod cladding. Such debris can include, for example, extraneous materials left over from reactor construction and various other materials employed during outages and repairs. It will be appreciated that the coolant moderator circulation system in a nuclear reactor is closed and that the debris accumulates over time with increasing age and use of the reactor. Many and various types of debris filters or catchers have been proposed and used in the past. One such system employs a series of curved plates extending substantially parallel to the direction of coolant flow interspersed with the webs and bosses of the lower tie plate grid to filter debris. While certain advantages accrue to this type of debris catcher, the various parts are difficult to manufacture and require complex assembly. Another type of debris filter uses a stacked wire concept perpendicular to the coolant flow. While this is effective in filtering out debris, the wires of the debris filter themselves have been known to generate debris, resulting in fuel bundle failures.
In current BWR debris filters of the assignee hereof, the debris filter is cast integrally with the lower tie plate. The hole size and small ligament web between the holes, however, are very near the investment casting manufacturability limits and oftentimes require hand rework to produce the filter. Particularly, an integral cast plate containing multiple holes extending parallel to the direction of coolant flow at the bottom of the boss/web structure of the lower tie plate grid supporting the fuel rods has been employed as a debris filter. While this design is simple and robust and does not add additional piece parts to the lower tie plate, any reduction in size of the debris filtering holes would render the lower tie plate very difficult to cast. It is therefore desirable to improve the effectiveness of a debris filter, while simultaneously improving its manufacturability and assembly, reduce its costs and provide a filter without substantially increasing the pressure drop and preferably decreasing the pressure drop to enable flexibility in the overall fine-tuning of the bundle thermal hydraulic design.
In accordance with a preferred embodiment of the present invention, there is provided a filter plate for overlying a cast lower tie plate having a grid formed of spaced bosses having holes and webs interconnecting the bosses defining with the bosses flow openings through the grid. The filter plate includes holes spaced one from the other for registration with the holes through the bosses when the filter plate overlies the grid. The filter plate also has a plurality of reduced apertures, e.g., reduced diameter apertures, spaced from one another and the holes through the filter plate. The apertures are significantly smaller in size and area than the holes. Additionally, the upper edges of the webs interconnecting the bosses are spaced below the upper edges of the bosses such that the filter plate rests only on the upper edges of the bosses and not along the upper edges of the webs. This frees additional apertures through the filter plate for flow of coolant therethrough.
To secure the filter plate in position on the lower tie plate grid, the fuel rods have either smooth-sided or threaded end plugs. The fuel rods having threaded end plugs constitute tie rods or part-length fuel rods. Those threaded end plugs extend through the filter plate holes and are threaded into corresponding female threaded holes of the bosses to secure the fuel bundle to the lower tie plate. However, the threaded end plugs do not engage the margins of the holes through the filter plate and do not assist in securing the filter plate to the tie plate. The smooth-sided end plugs of the remaining fuel rods extend through the registering holes of the filter plate and bosses and margins of the filter plate holes are engaged by those end plugs to maintain the filter plate positioned on the lower tie plate grid. The weight of the fuel rods with the smooth-sided end plugs hold down and position the filter plate against the lower tie plate.
The coolant flows through the lower tie plate and into the flow openings between the bosses and the webs. The flow continues through the apertures of the filter plate for flow about the fuel rods of the fuel bundle. The small size of the apertures through the filter plate serves to catch the debris in the coolant/moderator closed-circuit circulation system.
In a preferred embodiment according to the present invention, there is provided a nuclear fuel assembly comprising a fuel bundle having a plurality of fuel rods, a fuel rod support structure including a lower tie plate having an inlet nozzle, a lower tie plate grid and a transition structure for receiving coolant entering the nozzle and flowing coolant through the transition structure to the lower tie plate grid, the lower tie plate grid including a plurality of spaced bosses defining holes sized for receiving lower ends of the fuel rods within the holes of the bosses, the lower tie plate grid further including webs interconnecting the bosses to define with the bosses a plurality of flow openings through the lower tie plate grid for flowing coolant through the tie plate grid, a filter plate disposed on the upper edges of the bosses and having a plurality of spaced holes therethrough in registration with the holes in the bosses, the filter plate having a plurality of apertures therethrough spaced from the filter plate holes and in registration with the flow openings between the bosses and the webs, the area of each aperture being smaller than the area of each hole through the filter plate and the number of apertures being in excess of the number of the holes through the filter plate and a predetermined number of the fuel rods having end plugs received in the registering holes of the filter plate and the bosses and engaging the filter plate about the margins of the holes therethrough to position and maintain the filter plate against the ends of the bosses of the lower tie plate grid.
In a further preferred embodiment according to the present invention, there is provided a nuclear fuel assembly comprising a fuel bundle having a plurality of fuel rods, a fuel rod support structure including a lower tie plate having an inlet nozzle, a lower tie plate grid and a transition structure for receiving coolant entering the nozzle and flowing coolant through the transition structure to the lower tie plate grid, the lower tie plate grid including a plurality of spaced bosses defining holes sized for receiving lower ends of the fuel rods within the holes of the bosses, the lower tie plate grid further including webs interconnecting the bosses to define with the bosses a plurality of flow openings through the lower tie plate grid for flowing coolant through the tie plate grid, the webs having upper edges recessed below upper edges of the bosses, a filter plate disposed on the upper edges of the bosses and having a plurality of spaced holes therethrough in registration with the holes in the bosses, the filter plate having a plurality of apertures therethrough in registration with the flow openings between the bosses and the webs, the cross-sectional area of each filter plate hole being at least fifteen times the cross-sectional area of each aperture through the filter plate and the number of apertures being in excess of ten for each hole through the filter plate and a predetermined number of the fuel rods having end plugs received in the registering holes of the filter plate and the bosses.